Authors: Steven R. Cranmer (University of Colorado Boulder)

The Alfven surface denotes the location(s) where the solar wind’s outflow speed exceeds the phase speed of Alfvenic and fast-mode waves, and in several different ways it acts as a convenient boundary between the corona and heliosphere. It is becoming increasingly apparent that this region is sufficiently turbulent that there usually are multiple (stochastic and time-dependent) crossings of the Alfven surface along any radial ray. Thus, in many contexts, we now tend to think of this region as a “frothy Alfven zone” rather than an isolated radial boundary. This session will highlight recent discoveries about this critical region, from both new observations (e.g., Parker Solar Probe and PUNCH) and new insights gained from theoretical modeling and simulations. In this scene-setting talk I will attempt to review the current states of the observations and theory, and look ahead to results that may be coming around the corner very soon. If there is time, I may also point to a few nonlinear aspects of solar wind physics (e.g., blobs, switchbacks, reconnection events) that may not respect our usual expectations about what ought to remain on this or that side of the Alfvenic boundary.